Influence of decontamination and preconditioning on corrosion layer

Radiation field exists in nuclear power plants is primarily due to the deposition of radioisotopes on the surfaces of pipes and other primary components. These radiation fields cause occupational radiation exposure (ORE) to personnel engaged in maintenance work during refuelling shutdowns and thus significantly influence the operation and maintenance works on nuclear power plants. Dissolved and particulate corrosion products can also deposit on fuel cladding and primary system surfaces. Primary problems caused by fuel assemblies' deposits are the increase of cladding temperature, which enhances corrosion risk and may lead to and/or contribute to fuel rod failure, For VVERs the deposition mechanism is most likely influenced by some organic substances (residues from decontamination agents), whose behaviour in the active zone and role in the deposition mechanism are not completely known. Operational experience from various NPPs (e.g. Novovoronezh, Loviisa, and Paks) revealed the large impact of decontamination processes on the quality of oxide layer and deposits, so did the loop and autoclave tests. Actual in-pile loop tests carried at the Nuclear Research Institute (NRI) Rez are focused on the study of surface preconditioning and decontamination solutions' effect on surface layer after irradiation exposition. Effects of the decontamination on depositi formation onto primary circuit surfaces are investigated under steam generator (SG) operating conditions with the model device which contains SG heat exchanger tube, VVER spacer grids and heating rods simulating fuel cladding surface. The entire experiment is performed in experimental reactor water loop (RVS 4) on the NRI research reactor LVR-15. Oxide layer was built-up on the inner surface of as received SG tubes under higher temperature primary water conditions and with irradiation. This long-term exposure should enable to create oxide surface layers corresponding to the real conditions. The whole loop experimental program was divided into four categories to achieve comparison with different organics content and different mode of surface preconditioning after decontamination. The following specimens have been used for the tests: (i) as-received, (ii) after preconditioning and (iii) after exposition under NPP Paks operation conditions (real tube from NPP Paks). (author)